Cholesterol is one of the most intensely studied of molecules that circulate in the human bloodstream. Cholesterol is a lipid that is a major component of cell membranes and also is the precursor of steroid hormones and the bile acids. Two sources of cholesterol are available to cells. Endogenous cholesterol is synthesized in the liver and other cells and transported through the bloodstream to other cells. Since cholesterol is highly apolar, it is transported through the bloodstream in the form of lipoproteins consisting essentially of a core of apolar molecules such as cholesterol surrounded by an envelope of polar lipids, primarily phospholipids. Alternatively, exogenous cholesterol may be absorbed from the gut. Exogenous cholesterol is transported from the lumen of the gut into the blood or lymph for distribution via lipoprotein particles to other cells of the body.
For the diagnostic purposes related to human health, the lipoproteins are classified into several categories based on the density of the lipoprotein particles. The two categories most discussed in connection with human health are the low-density lipoproteins (LDL) and the high-density lipoproteins (HDL). For many people, HDL is known as the “good cholesterol” since it has a somewhat protective effect on the tendency of LDL to contribute toward coronary artery disease and related cardiovascular conditions such as stroke. Studies have shown an inverse relationship between levels of serum HDL and the occurrence of coronary artery disease, resulting in HDL levels being graded as a strong risk factor for cardiovascular disease prediction. Accordingly, a low level of HDL cholesterol, referred to as hypoalphalipoproteinemia, is a blood abnormality that correlates with increased risk of cardiovascular disease.
One rare form of genetic HDL deficiency is known as Tangier disease. Patients with the homozygous form of this disease have an almost total absence of serum HDL cholesterol. The disease is an autosomal recessive trait, and patients with the disease accumulate cholesterol esters in several tissues, resulting in characteristic physical features including enlarged orange or yellow tonsils, hepatosplenomegaly, peripheral neuropathy, and cholesterol deposition in the rectal mucosa. The symptoms of the disease appear to be attributable to a deficiency in cholesterol and/or phospholipid transport across cell membranes, principally out of cells that manufacture or store excess cholesterol. The orange tonsils are, for example, caused by the accumulation of cholesterol esters and related carotenoids in macrophages. It has now been established that Tangier Disease is a monogenic disorder caused by a mutation in the ABC1 gene (Brooks-Wilson, A. et al. 1999, “Mutations in ABC1 in Tangier disease and familial high-density lipoprotein deficiency.” Nat. Genet. 22:336–345; Bodzioch, M. et al. 1999, “The gene encoding ATP-binding cassette transporter 1 is mutated in Tangier Disease.” Nat. Genet. 22:347–351; Rust, S., et al. 1999, “Tangier Disease is caused by mutations in the gene encoding ATP-binding cassette transporter 1.” Nat. Genet. 22:352–355). Other patients exhibit a more common form of genetic HDL deficiency which results in low plasma HDL and premature cardiovascular disease, but an absence of the severe symptoms associated with Tangier disease. A large sub-group of patients with low HDL have an inherited form of this disease, familial hypoalphalipoproteinemia (FHA). It has been found that many of these patients are heterozygotes for mutations in ABC1. (Brooks-Wilson, A. et al. 1999, supra). Thus, ABC1 in its homozygous form causes Tangier disease and in its heterozygous form causes FHA.
An animal model for low HDL conditions exists in the form of the Wisconsin Hypo-Alpha Mutant (WHAM) chicken. This single gene mutation arose naturally and was identified because of the white skin phenotype and a closed flock of the chickens has been maintained as an animal model for low HDL disease. (Poernama et al. Jour. Lipid Res. 31:955–963 (1990)). The effect of this mutation on diet-induced atherosclerosis has been investigated, and it has been found that WHAM chickens are highly deficient in their ability to transport cholesterol from the gut into the blood. (Poernama et al. Arteriosclerosis and Thrombosis 12:2:601–607 (1992)). Some efforts have been made to identify the genetic element responsible for the mutation in the WHAM chickens (Schreyer et al. Arteriosclerosis and Thrombosis 14:12:2053–2059 (1994)), but prior to the instant invention, these efforts have not been successful.
It is highly desirable to identify and develop compounds and therapeutic agents which are useful for reducing cholesterol transport from the gut to the blood or lymph and for the regulation and treatment of cardiovascular disorders (such as high LDL or serum cholesterol levels), obesity, elevated body-weight index and other disorders relating to lipid metabolism.